1. Field of the Invention
The present invention relates to implantable medical devices. More specifically, the present invention relates to implantable medical devices that are capable of delivering pacing stimuli.
2. Description of the Related Art
At a superficial level, the mechanical aspects of the cardiac cycle of the human heart are fundamentally simple. The heart has four chambers. Deoxygenated blood is returned from the body to the right atrium. The right atrium (RA) fills the right ventricle (RV), which, upon contraction, pumps blood to the lungs. Oxygenated blood from the lungs fills the left atrium (LA), which in turn fills the left ventricle (LV). The contraction of the left ventricle then delivers oxygenated blood throughout the body. Thus, the atrial chambers serve the purpose of filling their respective ventricular chambers.
Similarly, the electrical and timing aspects of the cardiac cycle are also fundamentally simple, at a superficial level. The sinoatrial node (SA node) is the heart's natural pacemaker and initiates electrical depolarization of the heart at a predetermined rate, based upon physiologic need. The SA node is located in the right atrium and upon activation, the atrial chambers respond to the depolarization by engaging in a muscular contraction. The depolarization wavefront eventually reaches the AV node, where a delay is imparted before depolarizing and contracting the ventricles.
The cardiac cycle is often described by atrial (A) events and ventricular (V) events. Thus, the activation of the SA node is an intrinsic atrial depolarization. Some time later, the ventricles depolarize. There is a delay and the process is repeated. Thus, normal timing is A-V-A-V, etc. For purposes of understanding the physiology as well as for programming various pacemakers, this simple understanding provides several common variables. The rate of the heart is defined by a complete cycle and may either be an A-A interval or V-V interval (A-A will be used herein for explanatory purposes). The time between the atrial event and the ventricular event is the AV interval and not surprisingly, the time between the ventricular event and the subsequent atrial event is the VA interval.
As rate is increased, the A-A interval decreases in duration. The AV node modifies the delay imparted, thus the AV interval is also reduced. The mechanical actions involved (contraction of a chamber; ejection of a fluid) may occur more quickly, but there is a limit or minimal time required for efficacious operation.
This highly simplified overview can actually provide for many of the key programming parameters in a given dual chamber pacemaker. Typically, a dual chamber pacemaker will have an atrial lead (and electrode) positioned within the right atrium and a ventricular lead (and electrode) positioned within the right ventricle, generally with the electrode positioned at the right ventricular apex. Assuming a given patient had no intrinsic rhythm and fully relied upon the pacemaker, the rate would always be the device's escape interval which defines an A-A interval. This interval may be varied by the device based upon sensor input to provide rate responsive (RR) pacing. An AVI or AV interval is programmed and may be varied by the device depending upon rate or other factors. AV synchrony is maintained in that a ventricular paced event (VP) will always follow an atrial paced event (AP). A typical DDD pacemaker may operate in this manner.
While the present discussion is overtly superficial both in terms of the cardiac cycle and operation of a pacemaker, several fundamental aspects have been illustrated that are currently being questioned. The first is that ventricular pacing in the right ventricular apex may not be hemodynamically optimal for all patients. The second is that a programmed AV interval that more or less assures ventricular pacing, even to maintain synchrony, is not necessarily optimal in all patients. Finally, the entirety of the above discussion was in terms of RA to RV electrical timing, which while common parlance tends to ignore a great many aspects of the cardiac cycle.
Implanting leads into the right atrium and right ventricle is significantly easier than implanting leads to pace the left atrium or left ventricle, as leads on the left side are preferably implanted epicardially or within the veins of the heart proximate, but external to the relevant left sided chamber. That is, there is a general medical bias against placing leads or electrodes within the left atrium or left ventricle as this could promote clotting that results in a thrombus. Thus, right side implantation of single or dual chamber pacemakers is the norm. When dual chamber pacemakers are so implanted, the device is typically programmed to operate in a DDD mode or VDD for a single chamber, ventricular pacemaker. Such settings restore rhythm, but ensure that pacing occurs in a high percentage of cardiac cycles.
Ventricular pacing from the right ventricular apex causes the depolarization wave to travel a rather unnatural path and while it will cause the left ventricle to depolarize, the timing of the left ventricle with respect to the right ventricle is skewed electrically and mechanically. Recently, there has been recognition that intrinsic conduction is preferable to pacing in most cases. That is, even if the AV delay is longer than “normal,” it is preferable to wait for the intrinsically conducted beat than to pace. This is, of course, at odds with standard DDD (or similar) modes, which will provide a ventricular pace after a predetermined interval (AVI), which is usually short enough that it precludes intrinsic conduction. Certain patients who are pacemaker dependant, e.g., those that have complete heart block, will require and benefit from such ventricular pacing. Other patients may have pacemakers implanted for other reasons and have intact conduction or may have intermittent block. For those patients, intrinsic conduction is often if not always possible and is typically precluded by standard DDD mode settings.
Again referencing a device having leads in the right atrium and/or right ventricle, the timing relied upon both for programming/discussion purposes as well as what is sensed by the implanted device is based upon right side electrical timing. The use of right side timing will tend to ignore the delays in left sided response that occur naturally and/or as the result of pacing. In a normal, healthy heart the SA node will depolarize and generate a wavefront along an atrial conduction pathway that eventually reaches the left atrium causing it to depolarize and contract. The wavefront also reaches the AV node and progresses along the Bundle of His. The left sided pathway propagates somewhat faster than the right, but because the right ventricle is smaller the wavefront leads to a generally synchronized mechanical contraction of both ventricles.
When atrial pacing is introduced, the electrode is typically offset from the SA node, commonly in the right atrial appendage, and different conduction (and possibly less efficient) pathways are taken. See U.S. Pat. No. 5,179,949, issued to Chirife, which is herein incorporated by reference in its entirety. The net result is that there is an interatrial conduction delay (IACD) that is imposed. That is, the left atrium will depolarize and then contract after a longer interval from the pacing pulse than would occur intrinsically, i.e., after the SA node initiates depolarization. Thus, if the remainder of the conduction pathway were intact, this would skew the results for ventricular sensing. That is, an A pace occurs and after some interval, ventricular depolarization is sensed in the right ventricle by the pacemaker. This duration is determined to be the AV interval. However, the left atrium did not depolarize simultaneously with the A pace, nor within the normal physiologic window. Thus, the left sided AV (LAV) interval is shorter than the sensed right sided (RAV) interval. That is, LAV=RAV−IACD.
Another left sided variation to timing occurs when right sided ventricular pacing, particularly at the right ventricular apex is provided. As indicated, normal ventricular conduction begins at the AV node and more or less simultaneously propagates along a left and right side of the Bundle of His and spreads around each ventricle. With right sided pacing, the normal conduction pathway is not necessarily activated and instead propagation from cell to cell may occur at a slower rate. In addition, the wavefront propagates from the apex retrograde along the Bundle of His, then down the left side pathway eventually depolarizing the left ventricle. The delay imparted from the ventricular pace to contraction of the left ventricle is referred to herein as the interventricular conduction delay (IVCD).
Yet another offset is the difference between an event, e.g., an atrial depolarization, and the time at which that event is sensed by the pacemaker. This delay is referred to as the P wave sense offset (PSO).
These various delays are biased towards right side events. That is, failing to account for such delays may have the most consequence on left side activity, which is generally more important to hemodynamic performance.